DNA: Nature’s curling iron?

To identify as a mammal, a creature must have hair. Scientists aren’t sure how it evolved, but it’s been speculated that hair is a modified version of reptilian and fish-like scales that mammals developed to help them adapt to life on land. For humans, hair may have once served an important role in helping our ancestors adapt to new environments; for example, hair helps us regulate our internal body temperature by insulating body heat in cold climates, and regulating perspiration in hot ones. Whether a person has straight or curly hair can be partially attributed to their DNA, but the science behind curly hair isn’t exactly straightforward.
Some people try in vain to get their hair to curl, while others struggle to straighten it out—a battle that may lie in our genetics. Evidence indicates that some populations of people are more likely to have curly hair if they inherit specific changes in their DNA sequence1. One of these changes occurs in the DNA sequence coding for the gene TCHH, which produces a protein known as trichohyalin. Depending on the version of this gene that you inherit (along with multiple other genes), you may be more likely to have curly hair rather than straight hair2.

Like this article? Check out some of our other writings, including articles about the genetics of baldness and the ways DNA may affect facial hair growth patterns

But to understand how these genes might affect hair structure, we need to talk about what hair actually is: a long shaft made of protein, mostly a rigid structural protein called keratin. Long fibers of keratin are strung together and pushed out of the skin in structures that we know as hair. Typically, hair is broken down into two main units—the shaft and the hair follicle3. Hair follicles are little packages of specialized cells that each contribute to building a hair shaft. Within the follicles, some cells produce large amounts of keratin and then die. When these cells die, the keratin within them remains in place like a molding of the cell. These keratin molds are densely packed together, but they need to be linked with one another to give the hair strength, similar to how bricks need to be sealed together with mortar3,4.
Hair follicle cells use the protein trichohyalin as a temporary molecular mortar of sorts, linking fibers of keratin to each other (as well as other structural proteins). The keratin shaft is then pushed out of the skin when cells below it begin to replicate, thereby forcing the hair out of the skin. Just before it’s pushed out of the follicle, enzymes break down the trichohyalin which allows the hair to grow out4.
Scientists are still working out what causes hair to be curly1. Some evidence indicates that an asymmetrical hair follicle will result in uneven pressure from the replicating cells. This asymmetrical pressure may then cause the hair to curl and kink, giving it the curly look. Genetic research points to the trichohyalin protein as a contributing factor as well1. It’s not clear how changes in the TCHH gene might affect the protein structure or function, much less how these changes lead to hair curl (or lack thereof), but it’s likely related to trichohyalin’s role in anchoring the hair fiber to the follicle. You can imagine how changes in the ability to evenly apply and remove this molecular mortar might affect the shape of the hair; however, we don’t yet know if these aspects are affected by the known changes in the TCHH gene.
Of course, your genes aren’t destiny, and there are plenty of tools that help us have whatever kind of hair we like. But the next time you reach for the curling iron or straightening brush, just remember: DNA might explain why you need it!

1Westgate, Gillian E., et al. “The biology and genetics of curly hair.” Experimental Dermatology, vol. 26, no. 6, 2017, pp. 483–490., doi:10.1111/exd.13347. Web. 29 Nov. 2017.
2Medland, Sarah E. et al. “Common Variants in the Trichohyalin Gene Are Associated with Straight Hair in Europeans.” American Journal of Human Genetics 85.5 (2009): 750–755. PMC. Web. 29 Nov. 2017.
3Maderson, P. F. A. “Mammalian skin evolution: a reevaluation.” Experimental Dermatology, vol. 12, no. 3, 2003, pp. 233–236., doi:10.1034/j.1600-0625.2003.00069. Web. 29 Nov. 2017.
4Alibardi, Lorenzo. “Perspectives on Hair Evolution Based on Some Comparative Studies on Vertebrate Cornification.” Journal of Experimental Zoology Part B: Molecular and Developmental Evolution, vol. 318, no. 5, 2012, pp. 325–343., doi:10.1002/jez.b.22447. Web. 29 Nov. 2017.